פירוש על עבודה זרה 150:13

In the biblical passage from which these laws are derived, the Israelites had acquired title to the utensils (they took them as booty in war). Possession means that they must be immersed. But if one borrows the vessels, they need not be immersed.
We should note that there is something a bit strange in this law. If the requirement has to do with purity or with kashrut, then what should it matter if the vessel was bought or borrowed?